Conventionally, there has been a capacitive sensor in which a structure having a movable electrode supported by a fixed portion through an elastic element is formed, the movable electrode can move toward and away from the fixed electrode according to an external force, and a capacitance between these electrodes is detected, thereby detecting various physical values such as acceleration and angular speed (see Patent Document 1). As such a capacitive sensor, there is also a known capacitive sensor capable of detecting a physical value in a vertical axis direction by one mass portion that is displaced by a physical value such as acceleration (see Patent Document 2 and Patent Document 3).    Patent Document 1: Japanese Patent Application Laid-open No. 2000-28634.    Patent Document 2: Specification of U.S. Pat. No. 4,736,629.    Patent Document 3: Specification of U.S. Pat. No. 6,000,287.
According to the capacitive sensor of Patent Document 1, the elastic element is formed as a beam that is spirally extending from a fixed portion, and a movable electrode that is movably supported by the fixed portion through the elastic element is displaced mainly in a direction extending along a surface of a sensor (semiconductor layer). According to a capacitive sensor of each of Patent Documents 2 and 3, an asymmetrical mass portion is supported by a torsion beam that extends from a fixed portion called an anchor portion in a horizontal direction symmetrically such that mass balance is lost, and a physical value can be detected by a positional displacement of the mass portion caused by a torsion beam according to a physical value added in the vertical direction.
According to Patent Document 2, such a capacitive sensor is formed by working a metal material. According to Patent Document 3, the capacitive sensor is formed by working a semiconductor substrate such as silicon using a known semiconductor process. When a device is formed by working silicon by a semiconductor process, since fine working can be carried out, it is possible to form a smaller and more precise capacitive sensor as compared with a case that a capacitive sensor is formed by working a metal material as in Patent Document 3.
In the case of a structure in which a movable electrode is movable supported by a fixed portion through a beam as in Patent Document 1, a stress generated in the beam is varied depending on a shape of the beam as the elastic element and the maximum acceleration applied to the sensor, but when a thin and long beam is provided when a sensor is made compact or a spring constant is set, a stress generated in the beam is prone to become great, and it becomes difficult to set a specification such as a displacement amount or a weight of the movable electrode to a desired value in some cases.
According to the capacitive sensor disclosed in Patent Document 3, since a single crystal silicon substrate is formed by crystal anisotropy etching, there is a problem that various portions such as an anchor portion are tapered, and a deficit or sticking of a member is generated by increase of device size or movement of a movable electrode. When the substrate is worked by the crystal anisotropy etching, there is problem that it is difficult to form a mass portion that enhances the detection sensitivity because the mass portion has a mass to some extent.
The present invention has been proposed in view of the above circumstances, and an object of the present invention is to reduce a stress of a beam, in a capacitive sensor having a movable electrode that is movably supported by a fixed portion through the beam. Another object of the present invention is to provide a capacitive sensor having a structure that can avoid a deficit or a sticking of a member generated by increase of device size or movement of a movable electrode.